How effective is Aikido?

This question can be interpreted in different ways.

As a fighting tool.

Speaking in strictly technical terms, it is very effective at stopping all brainless charging. If a man or woman just charges at them brainlessly, the proverbial "bull rush" tackle attempt, an expert Aikidoka will send them flying. Nine times out of 10 I can almost guarantee it. The majority of street encounters always involve agression by untrained martial artists, so in that respect Aikido can be effective, since most violent assaults involve some sort of bullrushing. The consistent thing about primitive minded criminal assailants, is that they always move forward brainlessly, and Aikido works from there. To a certain extent so does Judo.

However because the martial art is so leverage and physics reliant, it is extremely difficult to perfect, to use it effectively you are looking at a solid decade of training. Not only that, but Aikido's arsenal is extensive; see, practice in Aikido requires that you be a good attacker, or most of the trips and throws just won't work. The momemtum of an opponent, is what many Aikido throws use, and without that momentum that aggressive action the move just will not work. That means that in Aikido training, you learn all sorts of attacks derived from various Japanese martial arts; you even learn "football" style rushing which Aikido borrows from Sumo.

As a life tool.

Because of Aikidos focus on harmony with your attacker, it makes it very hard to learn and use effectively, as mentioned above, with a large amount of training involved.

But the benefit of this is that Aikido experts practice a harmony within, as well as with their opponents. You learn to avoid confrontation without having 'lost' to your opponent. While mastering a very effective fighting martial art, such as those use in MMA, teaches you how to hurt an opponent, it doesn't necessarily translate into a more fullfiled life, because the need to fight and prove yourself over others is maintained.

In Aikido you don't need to assert victory over an opponent, but harmonise with them.

In theory.